Supersoft X-Ray Sources Identified with Be Binaries in the Magellanic Clouds*

Abstract

We investigated four luminous supersoft X-ray sources (SSSs) in the Magellanic Clouds suspected to have optical counterparts of Be spectral type. If the origin of the X-rays is in a very hot atmosphere heated by hydrogen burning in accreted envelopes of white dwarfs (WDs), like in the majority of SSSs, these objects are close binaries with very massive WD primaries. Using the South African Large Telescope, we obtained the first optical spectra of the proposed optical counterparts of two candidate Be stars associated with SUZAKU J0105–72 and XMMU J010147.5–715550, respectively, a transient and a recurrent SSS, and confirmed the proposed Be classification and Small Magellanic Cloud membership. We also obtained new optical spectra of two other Be stars proposed as optical counterparts of the transient SSS XMMU J052016.0–692505 and MAXI J0158–744. The optical spectra with double-peaked emission-line profiles are typical of Be stars and present characteristics similar to many high-mass X-ray binaries with excretion disks truncated by the tidal interaction with a compact object. The presence of a massive WD that sporadically ignites nuclear burning, accreting only at certain orbital or evolutionary phases, explains the supersoft X-ray flares. We measured equivalent widths and distances between line peaks and investigated the variability of the prominent emission-line profiles. The excretion disks seem to be small in size and are likely to be differentially rotating. We discuss possible future observations and the relevance of these objects as a new class of SN Ia progenitors.